Yes, it can be stopped. All Tim Sweeney is to do is build another mobile hardware/software platform and set the rules for app distribution however he likes. That is the way to beat a monopoly.
That's a really simple view of the issue. You can't reasonably think even a large corporation such as Epic can achieve that. That is what makes Apple/Google a duopoly in the mobile computing space. At this point, without any state intervention, they can abuse their position of power to bully devs and consumer. I don't care that they had to "work hard" to get there or whatever; the fact is they now have too much power…
Just because a market is hard or expensive to get into doesn't mean it's not a market worth entering. The real crux is Epic doesn't actually want an open phone platform, they just want to be software rent-slurping middlemen for an arbitrary 'cut' of a pie.